1. Field of the Invention
The invention relates to a connection between a drill pipe made of carbon- or glass fiber-reinforced plastic and a tubular, preferably metal connector that is partially held in one end of the drill pipe and is cemented to it.
2. Description of the Related Art
Continuing use of the existing infrastructure on land as well as “off-shore” requires drilling deeper or further. There is a series of applications that would use horizontal distance drilling, i.e., to deliver more petroleum from an existing field from existing platforms and to increase the degree of petroleum displacement of the depots.
The horizontal distance drilling is largely limited by the weight and the “normal force” of the drill string that determines the friction forces. Here, the tensile loading or buckling is adversely affected, and wellbore hydraulics (flow velocity compared to pressure losses) is largely limited by the limited diameter selection.
In this connection, bores of up to a distance of roughly 12 km are the limit that can be reached with the currently predominant material “steel” and the drilling technique used at present. The situation is similar for deep bores where the inherent weight of the string limits the depth, and bores beyond 8 km entail the greatest risk. Aluminum as a drill string material has not solved this problem and is rarely used. Glass or carbon fiber-reinforced plastics (“composites”) are alternatives.
One major factor that impedes the use of CFK or GFK drilling columns is ensuring a durable connection between the CFK or GFK pipe and the steel connector.
A known CFK drilling column that tries to solve this problem is described in U.S. Pat. No. 5,332,049 A. With this drilling column, the drilling of curved bores with a short radius of curvature with a limited range is enabled. U.S. Pat. No. 5,332,049 A shows a method for transfer of loads between a CFK pipe and a metal connector. The connection is ensured by simultaneous use of cements and pins.
The type of fastening according to U.S. Pat. No. 5,332,049 A, however, shows a series of disadvantages. The use of cement and pins leads to loss of the drill string in case of failure, and thus there is no possibility of retrieving the drill string from the drill hole. There is no effective method for ensuring that the cement is uniformly distributed and uniformly loaded. Moreover, failure of the connection as a result of torsion loading automatically leads to destruction of the axial load bearing capacity of the drilling column and thus to separation of the connection. Due to the different material properties (GFK or CFK and steel), the strength of the connection is limited. The expansion range of the GFK or CFK material cannot be tolerated by the steel connector. This leads to the maximum shear stress (stress concentration) being located at the steel-CFK or CFK transition. An increase of axial loading leads to a rise of local shear stresses. Subsequently, local failure of the adhesive connection occurs, which in turn leads to the rapid spread of the fault up to complete failure of the connection. Efforts to increase strength by increasing the adhesive length are largely in vain, since the local stress concentration at the GFK-steel or CFK-steel transition is only minimally dependent on the adhesive length.
WO 99/17045 A shows an improved CFK drill string. One part of this development is the connection between the steel connector and the CFK drill string. This type of fastening likewise has some disadvantages. There is a high stress concentration on the groove base both for CFK pipe and also for the steel connector. Under dynamic stresses, this leads to crack formation, crack propagation, and finally to complete failure of the connection. The complex production method makes it almost impossible to integrate this method into a continuous process, for example the so-called “pullwinding.” To increase the axial bearing capacity, grooves were made on the connector surface. They lead to poorer contact between the CFK pipe and the steel connector. Ensuring tightness under high pressure is doubtful. Both the axial and also the tangential grooves lead to inevitable curvature of the carbon fibers; this leads to complex loads (complex stress state) on the fibers.